1. Field of the Invention
The present invention relates to a classification device for classifying toner particles for use in image forming methods such as electrophotography, electrostatic recording, electrostatic printing, and toner jet recording.
2. Discussion of the Background
Methods of manufacturing toner are broadly divided into pulverization methods and polymerization methods. A typical pulverization method includes, for example, processes of mixing a binder resin (for fixing a toner on a recording medium) and a colorant (for providing the toner with a color), optionally along with additives such as a charge controlling agent (for providing the toner with chargeability), a magnetic material (for providing the toner with transportability), a release agent, and a fluidizer; melt-kneading the mixture; cooling and solidifying the melt-kneaded mixture; pulverizing the solidified mixture into fine particles; classifying the fine particles by size and collect desired-size particles; and mixing the collected particles with an external additive to obtain a toner. Such a toner is generally used for image forming. The toner may be further mixed with a magnetic carrier to be used for two-component developing methods.
Recent toners have a small particle diameter for high-definition printing while including a low-melting point binder resin and a release agent (such as a wax) for high-speed printing. Such toner particles having a particle diameter of 2 μm or less generally have poor heat-resistant storage stability, and are likely to cause the problems called “background fouling” and “filming”. “Background fouling” is a phenomenon in which the background portion of an image is disadvantageously soiled with toner particles. “Filming” is a phenomenon in which toner particles disadvantageously form thin film thereof on a photoreceptor, etc. Therefore, it is better to remove toner particles having a particle diameter of 2 μm or less from the resultant toner.
FIG. 1 is an explanatory diagram illustrating exemplary processes for manufacturing a toner. A material 41 to be pulverized is repeatedly subjected to a closed-circuit pulverization process in a pulverization device 42 equipped with a classification device for removing coarse particles so that coarse particles larger than desired-size particles are repeatedly subjected to pulverization. The pulverization device 42 may be an airflow-type pulverizer or a mechanical pulverizer, for example. The material 41 which have been pulverized into particles are classified into desired-size particles 44 and fine particles 45 by a classification device 43. The classification device 43 may be a multisegment airflow-type classifier or a vertical air diverter, for example.
Classification devices for manufacturing toners are divided into impeller types and non-impeller types. Impeller-type classification devices include what is called a tandem toner separator (hereinafter “TTSP”), which is one of wind power classifiers. A typical TTSP performs classification by balancing centrifugal force generated from blades provided on a rotor and centripetal force generated from fan suction force of the rotor. The centrifugal force collects coarse particles, whereas the centripetal force collects fine particles from the periphery of the rotor. A typical TTSP includes a single casing, within which two impeller-type classification rotors are provided with one end of each of which being supported. The impeller-type classification rotors are capable of being driven by a motor, and the axes thereof are coincident. The first end of each of the classification rotors in the axial direction is covered with a cover disc without penetration hole, and the second end thereof is equipped with a cover disc with a penetration hole for discharging fine particles or middle-size particles. The two classification rotors are provided so that the surfaces of the first ends thereof face with each other. As a result, a minute drift space is formed therebetween in the axial direction.
To respond to recent demands for high image quality, a small-size toner with a narrow size distribution is sought because such a toner provides good dot reproducibility. Dots formed with such a small-size toner are unlikely to vary even after developing or transfer processes. Therefore, a toner manufacturing apparatus is demanded which can accurately classify particles by size, in other words, which can remove particles with a particle diameter of 2 μm or less and collect particles with a particle diameter of from 3 to 4 μm so that a resultant toner has an average particle diameter of about 5 μm with a high yield.
Additionally, toners are simultaneously required to be fixable at low temperatures from the viewpoint of reduction of consumption energy, to include a low-softening-point binder resin from the viewpoint of color mixing, and to include a release agent to be applicable to oilless fixing processes. Such toners may have adhesion or fusion property.
In particular, such a toner having the above-described properties and a particle diameter with 2 μm or less have poor heat-resistant storage stability, and are likely to cause the problems of background fouling and filming. Therefore, a classification device which can remove toner particles with a particle diameter of 2 μm or less is demanded.
A conventional TTSP has a problem that desired-size toner particles with a particle diameter of from 3 to 4 μm are also removed along with fine particles when the maximum peripheral speed of the rotor is about 65 m/s, especially when the toner particles are for use in two-component development or non-magnetic one-component development. This is because the weight of such toner particles is very small.
Even if the classification device disclosed in Japanese Patent Application Publication No. 2008-026457 is improved so that the rotor can rotate at a peripheral speed of 70 m/s or more, it is likely that toner particles as final products adhere to the spiral member due to centrifugal force generated by the rotor. Further, because the outlet part provided on the base of the outlet short pipe is narrow, emission efficiency is also reduced due to adhesion and fusion of the toner particles to the outlet part.
When a TTSP operates continuously, the following problems may arise.    1) Particles with a particle diameter of from 3 to 4 μm are classified into fine particles in large quantity and removed from the resultant product toner, resulting in deterioration of toner yield;    2) Toner particles stagnate and adhere to the inner surface of a spiral member. The adhered toner particles release therefrom and are immixed in the resultant product toner; and    3) The residence time of toner particles in a classification chamber is destabilized due to adhesion of toner particles, resulting in deterioration of classification accuracy.
Japanese Patent Application Publications Nos. 2004-198640, 2001-293438, 2006-293268 and 2008-161823, and International Patent Application Publication No. WO2004/088431 each disclose a classification device or a method of manufacturing toner using a classification device. However, these classification devices or methods are required to more improve their classification accuracy.